The present application is relevant to U.S. patent application Ser. No. 09/940,542 being filed by Keiji Tsukada, Tsuyoshi Miyashita, Akihiro Kandori, Daisuke Suzuki, and Kouichi Yokosawa, and assigned to the present assignee, based on Japanese Patent Application No. 2001-044424 filed on Feb. 21, 2001, and is relevant to U.S. Patent Application Serial No. 09/940,507 being filed by Kouichi Yokozawa, Daisuke Suzuki, Keiji Tsukada, Tsuyoshi Miyashita and Akihiko Kandori, and assigned to the present assignee, based on Japanese Patent Application No. 2001-044426 filed on Feb. 21, 2001. The contents of those applications are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a magnetic field shielding apparatus for measuring a magnetic field generated from an examination-target with the use of a plurality of magnetic-flux meters including high-sensitivity SQUIDs (i.e., superconducting quantum interference devices), and a magnetic field measuring method and a magnetic field measuring apparatus using the shielding apparatus. More particularly, it relates to a magnetic field shielding apparatus for measuring a biomagnetic field generated by the activity of a cardiac muscle in a living body or the like, and a biomagnetic field measuring method and a biomagnetic field measuring apparatus using the shielding apparatus.
2. Description of the Related Art
There has been reported a magnetic shield that is manufactured by locating a plurality of different-diameter cylindrical shields concentrically in sequence and by forming a spacing between the respective cylindrical shields (the prior art 1: JP-A9-214166).
In the technology described in the prior art 1, the cylindrical shield is formed as follows: Using a magnetic shielding material composed of a 0.1 to 0.5 mm-thick and 20 to 50 cm-wide tape-shaped Permalloy, the magnetic shielding material is wound in a spiral-like manner around a light-weighted plastic cylinder that is employed as the core. Then, the overlapped portion where the magnetic shielding material has been superposed without a spacing is set to be 5 to 10 cm wide, thus forming the cylindrical shield. The magnetic shielding material is wound in the spiral-like manner so that the overlapped portion is formed into substantially 2 to 5 layers and the total thickness thereof becomes equal to an order of 0.5 to 3 mm. Also, in the overlapped portion, there are provided rivets with a 30-cm interval in the circumferential direction""s length. The overlapped portion is tightened and fixed by the rivets, and the contact surfaces in the overlapped portion have become contacts made between the metals.
Incidentally, the prior art 1 has described, as a conventional technology, the manufacturing of a magnetically shielded room like a prefabricated room with the use of many sheets of boards formed of the Permalloy, i.e., a Nixe2x80x94Fe-based high-permeability alloy material. According to the description, this conventional technology necessitates a long time to manufacture the magnetic shield and employs a large number of configuration components, thereby resulting in a problem that the magnetic shield becomes highly expensive. The price of the magnetic shield occupies a considerably large proportion in that of a biomagnetic field measuring appliance, and accordingly it has been desired to lower the price of the magnetic shield.
There has been reported the manufacturing of a light-weighted shielded room which is performed by using, instead of the Permalloy boards as the magnetic shielding material, a magnetic shield sheet formed by pasting together a soft-magnetic amorphous alloy film and a polymer film (the prior art 2: JP-A-2000-077890).
It is an object of the present invention to provide a light-weighted, small-sized, and high-performance magnetic field shielding apparatus at a low-cost with the use of a high-permeability sheet having a high-permeability, and to provide a magnetic field measuring method and a magnetic field measuring apparatus for measuring a magnetic field generated from an examination-target, in particular, a living body.
In the conventional technology in the prior art 1 where the shielded room is manufactured by combining the Permalloy boards, there existed the following problems: A problem of necessitating an annealing processing of the Permalloy after being processed, a problem of necessitating a wide area, a problem that the shielded room is heavy and its large weight imposes a limit onto a location where the shielded room should be set, the problem that the shielded room becomes highly expensive, and the like. In the prior art 2, as compared with the prior art 1 where the shielded room is manufactured by combining the Permalloy boards, the weight of the shielded room is reduced. This makes it possible to implement the lowering of the price; however, the problem of necessitating a wide area still remains unsolved. Accordingly, it has been desired to reduce the weight of the shielded room and to lower the price thereof even further.
A representative magnetic field measuring apparatus in the present invention includes the following units: A magnetic field shielding apparatus for shielding a component of an external magnetic field in a direction perpendicular to a first direction, a cryostat for maintaining a plurality of SQUID magnetic-flux meters at a low-temperature, the plurality of SQUID magnetic-flux meters detecting a component of a magnetic field generated from an examination-target in the direction perpendicular to the first direction, an apparatus for supporting the cryostat, a driving/detecting circuit for driving the SQUID magnetic-flux meters and detecting signals from the SQUID magnetic-flux meters, a calculation processing apparatus for collecting an output from the driving/detecting circuit and executing a calculation processing, and a display apparatus for displaying an output from the calculation processing apparatus.
The magnetic field shielding apparatus in the present invention is formed by locating a plurality of non-magnetic cylindrical members having a hollow portion in such a manner that the cylindrical members surround the axis of the first direction concentrically. A plurality of high-permeability sheets having a high-permeability are located in a state of being pasted on the surface (the inner surface and/or the outer surface) of each cylindrical member so that the high-permeability sheets are overlapped with each other partially.
A cylindrical member located on the innermost-side has a first opening at one end of the first direction and a second opening at the other end of the first direction. There is formed a third opening that penetrates the plurality of cylindrical members in the direction perpendicular to the axis of the first direction. The component of the external magnetic field in the direction perpendicular to the first direction is shielded in an inner-side space of the innermost-side-located cylindrical member. In the case where the examination-target is a living body, a living-body holder (i.e., a bed or a chair) for holding the living body (i.e., the subject) in a manner of making the body-axis direction of an examination-target area substantially parallel to the axis of the first direction is located in the inner-side space of the innermost-side-located cylindrical member.
The cryostat is partially inserted into the third opening, and the bottom plane of the cryostat is located in the inner-side space. A configuration is employed where the diameter of the third opening is made smaller than that of the bottom plane of the cryostat so that the smaller-diameter portion of the cryostat is inserted into the third opening. This configuration makes it unnecessary to enlarge the diameter of the third opening, thereby making it possible to enhance the magnetic field shielding ratio for the external magnetic field. In the case where the examination-target is the living body, the third opening is opposed to a chest plane or a back plane of the living body (the subject), and the bottom plane of the cryostat is located in the above-described inner-side space in a manner of being opposed to the chest plane or the back plane of the living body.
A position adjusting apparatus adjusts the position relationship between the bottom plane of the cryostat and the surface of the examination-target, adjusting the position relationship in the direction perpendicular to the first direction. The position adjusting apparatus is capable of changing, in the second direction perpendicular to the first direction, the position of the cryostat and the position of the holder for holding the examination-target. Moreover, the position adjusting apparatus is capable of changing the position of the holder in a third direction perpendicular to the first direction and the second direction, and also in the first direction.
Furthermore, in the magnetic field shielding apparatus, a fourth opening is formed which penetrates the plurality of cylindrical members in the direction perpendicular to the axis of the first direction. In the case where the examination-target is the living body, the fourth opening exists within the field-of-view of the subject, thereby giving a sense of openness to the subject.
The present invention makes it possible to implement, using the high-permeability sheet having the high-permeability, the light-weighted, small-sized, low-cost, and high-performance magnetic field shielding apparatus having the high magnetic field shielding ratio. Since the magnetic field shielding apparatus in the present invention is the light-weighted and small-sized one, the shielding apparatus requires no specific load-bearing capability to the location where the shielding apparatus will be set, and thus the shielding apparatus is capable of being set only if there exists just a small area. This eliminates the limit imposed onto the location where the magnetic field shielding apparatus, i.e., the magnetic field measuring apparatus, should be set.
Only from the measurement of a normal component of a magnetic field without measuring 2 directions of tangent components of the magnetic field, the magnetic field shielding apparatus in the present invention makes it possible to obtain a magnetic field distribution diagram where the peak pattern appears directly above an electric current source. As a consequence, it becomes possible to directly read the positions of a plurality of electric current sources in the examination-target, in particular, the living body. This, in the case where the examination-target is the living body, makes it possible to obtain data that is useful for the diagnosis of the heart diseases of an adult or a fetus. The magnetic field shielding apparatus in the present invention allows the magnetic field from the examination-target to be measured and to be displayed in a short time.